Conventional variable displacement hydraulic pumps have a rotating cylinder block with axially movable pistons which engage a tiltable swashplate for varying the stroke of the pistons. The displacement of the hydraulic pump is proportional to the stroke of the pistons within the cylinder block and, therefore, the tilt angle of the swashplate. When the swashplate is not tilted with respect to a "no flow" position, the pistons are not stroked and the pump has a zero displacement. The zero displacement position of the swashplate represents a neutral mode of operation of the pump.
In order to selectively prescribe the position of the swashplate, displacement controls are used to vary the swashplate position in response to a command input. Displacement controls take many forms, but in most cases they allow an operator to manually select a desired swashplate position and the corresponding hydraulic pump displacement.
Many displacement controls include a fluid-metering control valve having an axially movable spool which is displaced in response to a command input. Displacement of the valve spool away from an axially centered position results in the interconnection of output ports formed on the valve with a source of pressurized control fluid, such that the control fluid appropriately is metered to a servo mechanism for effecting an angular displacement of the swashplate. A command input typically is transmitted to the control valve through an input linkage having a remote manual input lever, such as a pivoted hand lever or a foot pedal. Rotation of the input lever, such as the rotation of a foot pedal in response to the application of a control force, causes an axial displacement of the valve spool and a resultant change in the position of the swashplate.
During the time that a control force is applied to the input lever, it is important that the control force be reacted so that a positive feedback force is developed to oppose rotation of the input lever and provide an operator with some measure of the amount of swashplate modulation which is occurring. The feedback force requirement commonly is satisfied by the use of a centering spring mounted on the control valve and interconnected between the valve spool and the control valve housing. Displacement of the valve spool generates an axial biasing force in the centering spring, such that the valve spool is biased toward a position representative of the no flow position of the swashplate. The centering force is transmitted through the input linkage to oppose the control force and urge the input lever to a position corresponding to the zero pump displacement position of the swashplate.
In order to close the servo control loop between the control valve and the swashplate, a feedback linkage interconnects the swashplate with the valve spool and is operative to convert the relative displacement of the swashplate and valve spool to a feedback force which counteracts the control force acting on the spool and holds the valve spool in a centered, steady-state fluid metering position.
It also is known to utilize a resilient override spring element in the input linkage to yieldably apply a force to the control valve in a manner which prevents an excessive manual force from being applied to the valve. In order to prevent excessive input force from being applied to the control valve during sudden control inputs, the override spring deflects and limits the force on the control valve. The force level at which the override spring deflects is a function of the stiffness and amount of initial compression of the override spring.
Deflection of the override spring also permits continued displacement of the input lever when the valve spool has reached a physical travel limit. The spring preload force must be sufficiently low to prevent excessive feedback force from being transmitted to the lever once a travel limit is encountered.
During operation of the pump, metal chips or particles may become lodged within the control valve and obstruct displacement of the valve spool. This is a particularly undesirable condition when the control valve is jammed in a position which holds the swashplate fixed in a maximum displacement condition. Input force can be applied rapidly to attempt to shear the particles and overcome the obstruction.
The use of an input lever-centering spring and an override spring in series presents a problem. In known devices, the torque which is applied to the input lever during normal operation of the device, that is, with the override spring not deflected, is determined largely by the stiffness of the centering spring. Normally, the force generated by the centering spring is just high enough to overcome friction between the valve spool and the valve housing and to center the input lever.
In some vehicle applications of hydrostatic pumps, however, it is desired to have a much higher centering spring torque on the input lever to provide greater force feedback to an operator of the input lever. If the centering spring stiffness is increased, the increased force level must be transmitted through the override spring, requiring that the torque level of the override spring be increased to prevent the override spring from deflecting during normal operation of the control. This requirement places a penalty on the control design in terms of the space required to house the override spring and in an increased input torque when the override spring deflects.
Another problem which exists in known devices is the inability to easily align the centered position of the valve spool with the corresponding "neutral" positions of both the input lever and the swashplate. The input linkage and the feedback linkage commonly are connected to the valve spool by means of a summing link, with the feedback linkage including a first link pivotally connected between the summing link and the swashplate, and the input linkage including a second link pivoted between the input lever and the summing link. The summing link, in turn, is pivoted to the valve spool. In order for the centered position of the valve spool to align with the corresponding neutral positions of both the input lever and the swashplate, the pivotal connections of the first and second links and the valve spool must lie along the summing link when the valve spool is in the centered position.
The alignment problem arises due to the fact that the centered position of the swashplate and the input lever are determined independently of the position of the summing link. The centered position of the swashplate is established during assembly of the pump and is maintained by a pair of swashplate centering springs symmetrically connected between the swashplate and the pump. The centered position of the input lever is also established by a centering spring, as previously described. Thus, the locations of the pivots on the first and second links specify the position of the summing link at the respective neutral positions of the input lever and swashplate. In order, then, to connect the summing link to the valve spool, it is necessary to adjust the axial position of the valve spool relative to the summing link and away from the centered spool position to line up the connection points on the valve spool and the summing link.
One approach to solving this problem has been to connect the summing link with the centered valve spool prior to connecting the input linkage with the summing link. The position of the summing link at a neutral condition then is determined by the centered position of the first link and the valve spool. The centered position of the input lever then is adjusted to conform with the prescribed summing link geometry. This proposal generates additional problems, however, in applications in which the input linkage includes an intermediate coupling which defines a non-linear response characteristic relationship between displacements of the input lever and the valve spool. By varying the neutral position of the input shaft, which in turn drives the coupling, the neutral position of the input lever is offset from the center of the predetermined response curve and results in unacceptable operating characteristics of the pump control.
This invention is directed toward overcoming the problems set forth above in a novel and useful way.